Okay, so I have thought, and drew this up. It is, basically, a combination of several types of valves I've gathered from this site and Nerfhaven.

At rest, the main chamber and barrel are sealed off. When the chamber pressurizes to, say, 100psi, so does the secondary chamber, by sliding the chamber piston forward. Once the pressures are even in both chambers, the chamber valve will reset via the spring. The barrel piston is connected to a pull rod. When the rod is pulled, the barrel piston opens the barrel. Because the barrel piston is sleeved over the chamber piston, the barrel piston will keep the chamber piston from opening, that is, until the force overcomes finger strength, which will actually reset the trigger. This, in turn, will cause the secondary chamber to refill.

I'm a little concerned about the force needed to open the barrel piston. Is there an easier way to overcome this, or should I not worry about it?

Before I get pummeled, I know I will have to make a good seal around the pull rod and the chamber. Problem solved.

Edit: In theory, if I use regulated air, I can pressurize both chambers to a certain pressure (for ease, I'll say 100psi). When it is fired, only the secondary chamber will be released, leaving 100psi still in the main chamber. This, in turn, will allow the secondary chamber to refill very quickly, say, as quickly as I can pull the trigger?

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Looks nice! I am not sure about the pressures and forces though. The pull rod would shoot back like in a piston. Not sure if these accelerations are really safe. But yeah for a semicannon it would be great.

Are you going to pull on the barrel piston to open it? If so then its going to be hard making it not leak. And if you are going to go through the trouble of not making leak why not make a triggered burst disk??

The biggest problem I see with this design is the force needed to open the barrel valve. Once it is open, however, it should stay open. The pull rod, of course, would be covered, to keep it from damaging anything.

Felex: I wanted a semi-auto type of valve. A triggered burst disk is not.